During space flights by the U.S. Space Shuttle, it is known that Primary Reaction Control System (PRCS) Thruster Engines have experienced failure. The cause of failure has been traced to moisture entering the oxidizer valves within the thruster engines. The cost of repairing one or two thrusters per flight is estimated to range from one-half to one million dollars. Each Shuttle has 38 thrusters, and additional failures could become a serious flight safety hazard.
U.S. Pat. No. 3,946,557 to A. W. Macbeth discloses a gas tight seal for a rocket motor nozzle using a plug in the throat of the nozzle. The plug is formed by filling an elastomeric bag with a plastic foam that may cured in situ to conform to the contour of the throat. After the foam has cured, the bag is sealed. This device, while recognizing the problem in leaving the nozzle unsealed, has a number of shortcomings which make it impractical for use with the Shuttle. Installation time and difficulty would be a problem because the Shuttle thruster nozzles need to be sealed almost immediately upon landing. Also difficulties are presented in servicing the thruster engine through the plug when such servicing requires evacuation or pressurization of the thruster engine. The Macbeth device discloses that servicing may be done through the plug, although no details are given and the disclosed device does not include this capability. No provision is made for pressures which may build up within the engine and suddenly expel the plug. Moisture may already be present within the sealed part of the thruster engine prior to emplacement of the plug, so that damage could occur in spite of an air tight seal. It is also not clear if the Macbeth plug is mechanically sturdy enough to ensure that the seal would remain in place when the Shuttle is transported via aircraft.
U.S. Pat. No. 4,773,474 to K. E. Stay discloses a snap-on fillerneck assembly for a tank which uses resilient fingers around a body to latch onto an interior neck, thereby holding the fillerneck assembly in place after the flexible fingers expand radially outward upon being pushed through the neck. A removable lock ring having a diameter approximately equal to the inner diameter of the body is inserted into the body and into engagement with interior ledges on the fingers, thereby blocking inward movement of the fingers. The Stay invention does not disclose how such a mechanism could be applied for use in sealing a thruster nozzle. The surface of the thruster nozzle typically has a coating which would be damaged if it were chipped or scratched by such flexible fingers. Also, while the thruster nozzle has a throat portion, the inner surfaces are rather smooth and present no sharp edges to provide latching in the manner disclosed in the Stay invention. Another problem with the Stay device is the absence of mechanisms for plug expulsion if the plug is accidentally left in place when the spacecraft is launched, while concurrently enabling the plug to maintain a seal at relatively high pressures when the engine was being serviced.
The disadvantages of the prior art are overcome by the present invention, and a relatively simple and cost effective thruster nozzle sealing system and apparatus are hereinafter disclosed for protecting rocket thruster engines during storage, transportation, and servicing of a spacecraft. Those skilled in the art have long sought and will appreciate the novel features of the present invention which solve these and other problems.